Water treatment by aluminum sulfate is the method most used in drinking water treatment plants to remove unwanted microorganisms. The inorganic coagulants are partially hydrolyzed salts. Their dissolution in water depends principally on the pH thereof. Under optimal conditions of pH and temperature, the floc forms a large readily settles heavy particle. However, it still remains after dissolution of aluminum residues. The determination of residual aluminum in treated water is determined by the so-called spiking method.\nThe treated water at the water treatment plant of the city of Skikda contain 210 mg / L residual aluminum with an average dose of 40 mg / L of aluminum sulfate, exceeding the WHO standards (200μg / L) and away from the recommended standards (100 ug / L). On jar-test conducted with laboratory flocculators under the same conditions, the residual aluminum obtained by the same method of dosage is 182 mg / L, 13.33% less than the WHO standards.\nLaboratory tests are also performed with a bentonite from M\'Zila (Algeria) with a dose of 3 mg / L and 40 mg / L of aluminum sulfate. The results show an amount of residual aluminum 167μg / L, 20.48% less than the standards.\nThe dry residues were characterized microscopy by electron scanning (MEB-EDX) to mount the different spectra of aluminum in dry residues. Crude bentonite is characterized by elemental chemical method using X-ray fluorescence